The present invention relates to a signal processor for a VTR (which is abbreviated from Video Tape Recorder as in the following) and, more particularly, to a technique which is effective if applied to a frequency converter for converting the reproduced video signals of a color under system such as the VHS, S-VHS or 8 mm video system into standard color signals.
In the home VTR, color video signals are recorded by the color under system. In this system, the luminance signals are frequency-modulated, and the color signals are frequency-converted to a lower frequency band than that of the frequency-modulated luminance signals and are recorded on the inclined track of a magnetic tape by a rotary video head.
The VTR of recent years uses the guard bandless system for a higher density and is essentially required to clear a noise (as will be called the "crosstalk component") caused by a crosstalk. The clearance of this crosstalk component is effected by giving an azimuth angle to the video head, and this azimuth angle is effective highly for high-frequency signals but lowly for low-frequency signals. In other words, the clearance of the crosstalk component is little effective for the color signals, and the phase shift system (i.e., the PS system) or the phase invert system (i.e., the PI system) is adopted for the effect.
A method of clearing the crosstalk component in the color under system will be described by using the NTSC format of the VHS system. The video recording track is recorded by alternating two tracks, i.e., Channel 1 and Channel 2. In the NTSC format of the VHS system, the color under frequency is set to forty times (40 f.sub.H) as high as the horizontal scanning frequency f.sub.H (f.sub.H =15.734 KHz). As a result, the standard color signals are recorded by converting their sub-carrier frequency of 3.579545 MHz into 40 f.sub.H, i.e., about 629 KHz. At this time, the phases are advanced for one horizontal period (1 H) by 90 degrees in Channel 1 and delayed by 90 degrees in Channel 2. This is because when the reproduced color under signal of 629 KHz is inversely converted at a playing time to 3.579545 MHz (while the phases are also returned), the reproduced color under signals delayed by one horizontal period (1 H) can be added to clear the crosstalk components by using a delay element of one horizontal period (1 H) and by using the reproduced color under signals before delayed and the delay element.
The frequency conversion technique we have investigated before our invention is shown in FIG. 12, FIG. 13 and FIG. 14. In FIGS. 12 and 13, the crosstalk component is cleared by converting the reproduced color under signals into the standard color signals of 3.579545 MHz with a main converter and then by delaying the standard color signals by one horizontal period (1 H) with a glass delay line or a CCD (i.e., Charge Coupled Device as in the following). In FIG. 14, the phase adjustment is omitted by delaying the low-pass reproduced color under signals by one horizontal period (1 H) or two horizontal periods (2 H) in the low frequency with the CCD and by adding it to or subtracting it from the reproduced color under signals before delayed.
The aforementioned frequency conversion technique is exemplified by Japanese Patent Laid-Open No. 257394/1988 or 51489/1990.